This invention relates to automatic gain control circuits. In particular, this invention relates to an automatic gain control circuit particularly suitable for processing the output of an optical detector converting optical high-definition television signals to electrical signals for transmission to a receiver.
Optical fiber has become a popular medium for the transmission of telecommunications signals. The speed and accuracy of optical signals allows for the transmission of large volumes of information over a single optical fiber, and with current compression techniques the capacity of optical fiber transmission systems is virtually unlimited. Thus, optical fiber transmission systems are particularly useful for high speed transmissions involving large concentrations of data, such as high-definition television signals.
However, telecommunications equipment such as transmitters and receivers still operate by processing electrical signals. In a typical digital television receiver, for example, data is input to the receiver circuitry as a series of electrical pulses. These pulses represent pixel image data such as chrominance and luminance, synchronization data such as horizontal sync, vertical sync and frame sync, along with other characteristics of the televised image and associated data such as audio, closed captioning etc. The electrical pulses occupy standardized positions within a telecommunications signal, which are thus decoded by the receiver circuitry to create an image on the television screen.
High-definition television (HDTV) has presented a considerable challenge to conventional telecommunications technologies, due to the volume of information required to construct an HDTV image. Fiber optic telecommunications technology provides a partial solution, since the data rate of optical fiber transmissions easily accommodates the 1.485 Gb/s data rate standard for HDTV transmissions. However, an opto-electric coupler interface is required which is capable of translating optical (typically laser-generated) pulses received from an optical fiber to corresponding electrical pulses which can be processed by an HDTV receiver.
One type of opto-electric coupler suitable for standard optically transmitted telecommunications signals is a laser optical receiver, which emits a low level electric pulse corresponding to each optical pulse sensed by an optical detector. Existing laser optical receivers are capable of resolving optical pulses only to a power level of approximately xe2x88x9215 dBm. However, SMPTE specification 292M for a laser optical receiver for HDTV applications requires that the sensitivity of the receiver extend to at least xe2x88x9220 dBm.
Conventional optical detectors tend to poorly process so-called xe2x80x9cpathologicalxe2x80x9d signals, i.e. signals having large numbers of consecutive zeros or ones, because of the high DC content and low frequency content resulting from the absence of zero-to-one and one-to-zero crossover points. Also, the output signal level from a conventional optical detector tends to fluctuate widely, while the DC restoration circuitry used to equalize the signal output from the optical receiver requires a relatively constant input signal in order to perform properly. Thus, conventional optical receivers are poorly suited for use in high-speed applications such as HDTV.
The present invention overcomes these disadvantages by providing an optical receiver having a sensitivity extending well below xe2x88x9220 dBm, and an automatic gain control circuit therefor. The gain control circuit of the invention is self-stabilizing and provides a consistently accurate and uniform output signal level to the DC restoration circuitry, even under pathological signal conditions.
The invention accomplishes this by coupling the low level signal produced by an optical detector to a signal amplifier, the gain of which is controlled by a negative feedback circuit. The feedback circuit comprises a signal level detection circuit coupled to the amplifier output, in the preferred embodiment comprising high-speed Schottky diodes acting in conjunction with an operational amplifier. The Schottky diodes are coupled to ground through AC bypass capacitors, and oriented in opposite directions. Thus, when the amplified signal exceeds a conduction threshold of the Schottky diodes the AC bypass capacitors are respectively charged and drained, establishing a voltage difference between the input terminals of the operational amplifier. A current output from the operational amplifier to reduce the voltage difference is also fed to the gain control pin of the signal amplifier, reducing its gain.
The circuit of the invention thus establishes a negative feedback loop with the differential amplifier, to maintain a constant differential signal output level. The differential output signal is equalized for transmission to switching, routing or processing equipment or to an HDTV receiver.
In the preferred embodiment the amplifier is a double-ended differential amplifier, with the optical detector output fed into the high input and the low input coupled to ground, and the signal level detection circuit coupled in parallel to the differential outputs. Also, in the preferred embodiment the Schottky diode circuit clips peak levels of the differential output signal whenever they exceed a saturation voltage, which further stabilizes the differential output signal and avoids overloading of the DC restoration circuitry.
The present invention thus provides an automatic gain control circuit, comprising a differential amplifier having a high input adapted to receive a high input signal, a low input adapted to receive a low input signal, a high output for outputting an amplified high output signal, a low output for outputting an amplified low output signal substantially symmetrical with the high output signal, and a gain control input for receiving a gain control signal which controls the gain of the differential amplifier, and a feedback circuit comprising a signal level detection circuit comprising a first pair of diodes having coupled anodes and having cathodes respectively connected to the high output and low output, and a second pair of diodes having coupled cathodes and having anodes respectively connected to the high output and low output, and an operational amplifier having an inverting input connected to the coupled anodes of the first pair of diodes and to a first end of a first capacitor, a non-inverting input connected to the coupled cathodes of the second pair of diodes and to a first end of a second capacitor, the other ends of the first and second capacitors being connected to ground, and an output connected through a resistor to the inverting input and connected to the coupled anodes of the first pair of diodes, wherein an increase in the level of the amplified output signals causes a current to be withdrawn from the first capacitor which lowers a voltage at the inverting input and causes a current to be injected into the second capacitor which raises a voltage at the non-inverting input, causing a voltage difference between the output of the operational amplifier and the inverting input, whereby current flowing from the output of the operational amplifier to the inverting input flows to the gain control input of the differential amplifier to reduce the gain of the differential amplifier.
The present invention further provides an optical receiver for converting an optical signal to an electrical signal, comprising an optical detector for producing an electrical input signal corresponding to an optical input signal detected by the optical detector, a differential amplifier having a high input adapted to receive the electrical input signal, a low input adapted to receive a low input signal, a high output for outputting an amplified high output signal, a low output for outputting an amplified low output signal substantially symmetrical with the high output signal, and a gain control input for receiving a gain control signal which controls the gain of the differential amplifier, and a feedback circuit comprising a signal level detection circuit comprising a first pair of diodes having coupled anodes and having cathodes respectively connected to the high output and low output, and a second pair of diodes having coupled cathodes and having anodes respectively connected to the high output and low output, and an operational amplifier having an inverting input connected to the coupled anodes of the first pair of diodes and to a first end of a first capacitor, a non-inverting input connected to the coupled cathodes of the second pair of diodes and to a first end of a second capacitor, the other ends of the first and second capacitors being connected to ground, and an output connected through a resistor to the inverting input and connected to the coupled anodes of the first pair of diodes, wherein an increase in the level of the amplified output signals causes a current to be withdrawn from the first capacitor which lowers a voltage at the inverting input and causes a current to be injected into the second capacitor which raises a voltage at the non-inverting input, causing a voltage difference between the output of the operational amplifier and the inverting input, whereby current flowing from the output of the operational amplifier to the inverting input flows to the gain control input of the differential amplifier to reduce the gain of the differential amplifier.
The present invention further provides an automatic gain control circuit, comprising an amplifier having an input adapted to receive an electrical input signal, an output for outputting an amplified output signal, and a gain control input for receiving a gain control signal which controls the gain of the amplifier, and a feedback circuit comprising a signal level detection circuit comprising a first pair of diodes having coupled anodes and having cathodes respectively connected to the output and to a reference voltage, and a second pair of diodes having coupled cathodes and having anodes respectively connected to the output and to the reference voltage, and an operational amplifier having an inverting input connected to the coupled anodes of the first pair of diodes and to a first end of a first capacitor, a non-inverting input connected to the coupled cathodes of the second pair of diodes and to a first end of a second capacitor, the other ends of the first and second capacitors being connected to ground, and an output connected through a resistor to the inverting input and connected to the coupled anodes of the first pair of diodes, wherein an increase in the level of the amplified output signal causes a current to be withdrawn from the first capacitor which lowers a voltage at the inverting input and causes a current to be injected into the second capacitor which raises a voltage at the non-inverting input, causing a voltage difference between the output of the operational amplifier and the inverting input, whereby current flowing from the output of the operational amplifier to the inverting input flows to the gain control input of the differential amplifier to reduce the gain of the differential amplifier.